Fix async iterator runtime type errors

pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/generators.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /// This library adapts ES6 generators to implement Dart's async/await.
 /// It's designed to interact with Dart's Future/Stream and follow Dart
 /// async/await semantics.
 /// See https://github.com/dart-lang/sdk/issues/27315 for ideas on
 /// reconciling Dart's Future and ES6 Promise.
 /// Inspired by `co`: https://github.com/tj/co/blob/master/index.js, which is a
 /// stepping stone for proposed ES7 async/await, and uses ES6 Promises.
 part of dart._runtime;
 
 final _jsIterator = JS('', 'Symbol("_jsIterator")');
 final _current = JS('', 'Symbol("_current")');
 
 syncStar(gen, E, @rest args) => JS(
     '',
     '''(() => {
   const SyncIterable_E = ${getGenericClass(SyncIterable)}($E);
   return new SyncIterable_E($gen, $args);
 })()''');
 
 @JSExportName('async')
 async_(gen, T, @rest args) => JS(
     '',
     '''(() => {
+  let FutureRaw = ${getGenericClass(Future)};
+  let FutureT = FutureRaw($T);
+  let FutureD = FutureRaw(dart.dynamic);
   let iter;
   function onValue(res) {
     if (res === void 0) res = null;
     return next(iter.next(res));
   }
   function onError(err) {
     // If the awaited Future throws, we want to convert this to an exception
     // thrown from the `yield` point, as if it was thrown there.
     //
     // If the exception is not caught inside `gen`, it will emerge here, which
     // will send it to anyone listening on this async function's Future<T>.
     //
     // In essence, we are giving the code inside the generator a chance to
     // use try-catch-finally.
     return next(iter.throw(err));
   }
   function next(ret) {
     if (ret.done) return ret.value;
     // Checks if the awaited value is a Future.
     let future = ret.value;
-    if (!$instanceOf(future, ${getGenericClass(Future)})) {
-      future = $Future.value(future);
+    if (!$instanceOf(future, FutureD)) {
+      future = FutureT.value(future);

[Jennifer Messerly, 2017/03/23 00:31:19]

Chatted w/ Vijay offline ... I don't think this is right, because this is legal
in Dart:

Future<int> f() async {
  var a = await 'hello';
  var b = await 'world'
  return (a + b).length; // 10
}


... so we need to have the final Future be typed Future<T>, while allowing the
intermediate ones to be Future<top type> ... it may be that we can't use our
implementation like this anymore and we need to track the "final" one separately
with a Completer. IIRC that's how dart2js implements it.

     }
     // Chain the Future so `await` receives the Future's value.
-    return future.then($dynamic)(onValue, {onError: onError});
+    return future.then($T)(onValue, {onError: onError});
   }
-  return ${getGenericClass(Future)}($T).microtask(function() {
+  return FutureT.microtask(function() {
     iter = $gen.apply(null, $args)[Symbol.iterator]();
     return onValue();
   });
 })()''');
 
 // Implementation inspired by _AsyncStarStreamController in
 // dart-lang/sdk's runtime/lib/core_patch.dart
 //
 // Given input like:
 //
@@ skipping 158 matching lines @@
   }
 ''');
 
 /// Returns a Stream of T implemented by an async* function. */
 ///
 asyncStar(gen, T, @rest args) => JS(
     '',
     '''(() => {
   return new $_AsyncStarStreamController($gen, $T, $args).controller.stream;
 })()''');